1. Field of the Invention
The present invention relates to an overdischarge preventing circuit apparatus for a secondary battery detachably attached to an electric apparatus or incorporated in an electric apparatus, and an overdischarge preventing method for a secondary battery.
2. Description of the Related Art
When a secondary battery which can be repeatedly used by charging is discharged below a predetermined battery voltage, the battery performance deteriorates, so the cycle life significantly shortens. Therefore, overdischarge of a secondary battery is prevented by using a method which detects the battery voltage of a secondary battery and, if this battery voltage falls below a predetermined voltage, shuts down discharge of the secondary battery. For example, in Jpn. Pat. Appln. KOKAI Publication No. 4-33271, a switching element is connected in series with a secondary battery and turned off if the battery voltage of the secondary batter falls below a predetermined voltage, thereby shutting down discharge. In this reference, the switching element is kept off until the battery voltage of the secondary battery becomes slightly higher than the predetermined voltage. When the battery voltage becomes slightly higher than the predetermined voltage, the switching element is turned on to resume discharge. That is, a hysteresis characteristic is given between the shutdown voltage and resumption voltage of discharge. For example, this circuit is implemented by using a Schmitt trigger circuit.
Nonaqueous electrolyte secondary batteries such as lithium ion secondary batteries have been mainly used in portable electronic apparatuses such as personal computers, cell phones, and PDAs. In these applications, a discharge current is the largest in a personal computer, a maximum of about 4 A, and causes intermittent discharge. Hence, overdischarge can be prevented by detecting the battery voltage.
Recently, it is being attempted to use this nonaqueous electrolyte secondary battery in an electric apparatus which requires discharge with a large electric current. Examples are an uninterrupted power supply (UPS) for a personal computer (PC) or server, a cordless cleaner, an electric tool, and an automotive electronic apparatus. To use a nonaqueous electrolyte secondary battery in such an electric apparatus, discharge must be performed continuously with a large electric current of, e.g., 10 A or 20 A. This increases the heat generation amount by Joule heat when the secondary battery supplies power (discharges). Accordingly, the method which stops discharge when a predetermined battery voltage is reached cannot prevent overdischarge and, in addition to that, may cause a secondary disaster, e.g., an electrolyte leakage or a heat deterioration of a battery pack, by heat generation.
The scope of claims of Jpn. Pat. Appln. KOKAI Publication No. 8-98422 describes that when the battery temperature rises to a predetermined value or higher, charge or discharge of a battery is stopped by a switching circuit. Also, paragraph [0006] of this publication discloses a structure, as a protecting mechanism against a battery temperature rise, which opens the circuit at 70° C. or more by inserting a thermal protector in a pack battery.
If, however, a nonaqueous electrolyte secondary battery is discharged at a high rate, the battery temperature rises abruptly. Therefore, in the method in which discharge is stopped whenever the same battery temperature, e.g., 70° C., is reached, discharge must be stopped while a large dischargeable battery capacity remains. This makes it impossible to satisfactory achieve the performance of the battery.